1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus. In particular, the present invention relates to a zoom lens having a high variable magnification ratio, and which is appropriate for use in a surveillance camera for monitoring a long distance area, and to an imaging apparatus including the zoom lens.
2. Description of the Related Art
Conventionally, zoom lenses were used as optical systems for surveillance video cameras to monitor or observe long distance areas at harbors or the like. The zoom lenses used for such a purpose need to have high variable magnification ratios of at least 50 times and high performance. Further, such zoom lenses need to cope with wavelengths in a near-infrared region to perform photography at night or in a dense fog. Further, since the zoom lenses are high magnification optical systems, it is desirable that anti-vibration functions are provided.
As the zoom lenses having high variable magnification ratios, and which are appropriate for video cameras for monitoring long distance areas, for example, U.S. Patent Application Publication No. 2006018035 (Patent Document 1) discloses a zoom lens having a variable magnification ratio of approximately 60 times. This lens system is composed of a positive first lens group, a negative second lens group, a positive third lens group, and a negative fourth lens group. Further, magnification is changeable by moving the second lens group and the third lens group along the optical axis of the zoom lens. Further, U.S. Patent Application Publication No. 2008239504 (Patent Document 2) discloses a zoom lens having a variable magnification ratio of approximately 20 times. In the lens system disclosed in Patent Document 2, aberrations, especially chromatic aberrations, are corrected in an excellent manner by appropriately setting the arrangement of refractive powers in the first lens group, Abbe numbers or the like.
The zoom lens disclosed in Patent Document 1 achieves a high variable magnification ratio. However, to satisfy a demand for a high-performance zoom lens of recent years, correction of aberrations, especially correction of chromatic aberrations at telephoto end, needs to be improved. Meanwhile, the zoom lens disclosed in Patent Document 2 has high performance, but the variable magnification ratio is approximately 20 times. Therefore, a higher variable magnification ratio is desirable when the zoom lens is used to monitor a long distance area in surveillance operations at harbors or the like. As described above, it is difficult to provide an optical system that has excellent performance through the entire variable magnification range while achieving a high variable magnification ratio, and that is usable in a wavelength band of visible light to near-infrared light. Further, the size of an optical system tends to become large to satisfy these requests at the same time. However, in recent years, the compactness of an apparatus became strongly requested.